The invention relates generally to an ink supply device and an ink tank mounted on a carriage carrying an ink jet type recording head thereon, and more particularly, to a ink supply device which maintains ink in a container at a constant negative pressure irrespective of the ink consumed by a recording head to thereby supply ink to the recording head with accuracy.
In a conventional ink jet printer comprises a carriage having an ink jet type recording head mounted thereon. The recording head applies pressure to a pressure generation chamber in communication with a common ink chamber and a nozzle opening to thereby cause ink drops to be jetted out from the nozzle opening. The recording head further includes an ink cartridge for supplying ink to the ink jet type recording head. In this manner, as the carriage moves in reciprocating directions, the ink drops are jetted out onto recording paper in response to the data to be printed.
In a previous ink jet printer, the nozzle opening of the recording head is situated at a position lower than the ink liquid surface of the ink cartridge. Therefore, a head pressure is applied to the nozzle opening. To deal with this pressure, a porous elastic member, formed of foam, rubber or the like, is stored within the ink cartridge and the pressure of the ink cartridge is set slightly lower than the nozzle opening due to the surface tension of the porous elastic member to prevent the ink from oozing from the nozzle opening.
This structure does not solve all the pressure problems. For example, if the consumption of the ink advances so that the quantity of the ink absorbed in the porous elastic member is reduced to a small quantity, then the surface tension of the porous elastic member is increased. Therefore, the supply of ink to the recording head is not stable so that the ink stored within the cartridge cannot be completely consumed. Also, because the quantity of the ink to be stored within the cartridge is reduced by a quantity corresponding to the substantial volume of the porous elastic member, the size of the ink cartridge must be increased to compensate for the increase in volume. Further, there is a possibility that air bubbles contained in the porous elastic member may flow into the recording head.
U.S. Pat. No. 4,794,409 was developed to overcome some of the above-mentioned problems. This patent discloses an ink supply system in which a porous member is compressed and inserted between an ink container, not in communication with the air, and a recording head in such a manner as to form capillary spaces in part of the porous member. Further, adjacent to the porous member, a cavity is formed to serve as an ink reservoir. Based on this construction, the ink of the ink container is discharged into the ink reservoir and the ink is supplied from the ink reservoir through the porous member to the recording head to apply a negative pressure to the recording head due to the capillary force of the porous member. However, since the ink container, ink reservoir and capillary spaces are all formed as closed areas, the ink cannot be supplied stably from the ink container to the recording head.
To solve the above problem, it is possible to provide a structure as shown in FIGS. 25(a), (b) and (c). Referring to FIG. 25(a), a fine tube C is connected to a bottom portion B of an ink container A. A space D of ink container A is open to an air port E of fine tube C. Ink is supplied from an ink supply port F of bottom portion B to an ink jet type recording head. Based on this structure, the air is allowed to enter from a bottom portion G of fine tube C to thereby generate air bubbles K before space D can be made to communicate with air port E. As shown in FIG. 25(b), the ink jet type recording head functions as a suction pump P when the pressure of space D of ink container A decreases to a level to overcome the capillary force of fine tube C.
Due to the above operation, it is possible to maintain space D of ink container A at a negative pressure so as to maintain the printing operation of the ink jet type recording head. However, in graphic printing or the like, where the recording head uses a large quantity of ink, the inflow of the air through fine tube C does not catch up with the quantity of the ink consumed by the recording head. Therefore, the negative pressure in space D of ink container A is increased so that the supply of the ink to the ink jet type recording head stops frequently, thereby interrupting the printing operation of the ink jet type recording head as shown in FIG. 25(c). In addition, if the liquid surface of the ink decreases to bottom portion G of fine tube C, then there is no capillary force in fine tube C. Thus, the negative pressure of space D cannot be maintained. As a result thereof, a large quantity of ink may be supplied, thereby causing the ink to leak out and air bubbles to invade into the ink jet type recording head.
Accordingly, it is an object of the invention to provide an ink supply device which can maintain ink in an ink container at a constant negative pressure regardless of the quantity of the ink consumed by a recording head to thereby supply the ink to the recording head with accuracy.